Surface emitting semiconductor laser and communication system using the same

ABSTRACT

A surface emitting semiconductor laser includes a laminate of semiconductor layers emitting multimode laser light, and a block member blocking light of a specific mode among the multimode laser light emitted from the laminate.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a surface emitting semiconductorlaser of selective oxidization type, and more particularly, to astructure for controlling the optical power of a multimode surfaceemitting semiconductor laser.

[0003] 2. Description of the Related Art

[0004] Recently, there has been an increased demand for a surfaceemitting semiconductor laser that can easily realize an arrangement of atwo-dimensional array of sources in the technical fields of opticalcommunications and optical recording. Such a surface emittingsemiconductor laser is also called a vertical-cavity surface-emittinglaser (VCSEL) The VCSEL has many advantages of a low threshold voltage,low power consumption, easy making of a circular spot of light and goodproductivity resulting from wafer-based evaluation.

[0005] Nowadays, low-cost multimode optical fibers, which are typicallyplastic optical fibers (POF), are developed, and short-distance opticalcommunications as short as a few meters to several hundreds of metersare getting the attention. Generally, long-distance opticalcommunications employ the combination of a single-mode type opticalfiber and an edge emitting semiconductor laser having a relatively longwavelength of 1.31 μm or 1.55 μm. However, these components areexpensive and are unsuitable for applications in local areas that shouldbe less expensive.

[0006] An optical source used for the multimode optical fiber isrequired to be less expensive and avoid the use of a special opticalsystem or driving system. The surface emitting semiconductor laser maysatisfy the above-mentioned requirements and is one of the powerfuloptions as the optical source for the multimode optical fiber. In orderto use the surface emitting semiconductor laser as the optical source inoptical communications utilizing the multimode optical fiber, it isdesired to stabilize the oscillation mode and reduce jitter componentscontained in the optical output.

[0007] Japanese Laid-Open Patent Application Publication No. 2001-156395discloses a selectively oxidization type surface emitting semiconductorlaser in which an optically transparent layer is formed in a beamemitting aperture for controlling the oscillation transverse mode. Thetransparent layer may, for example, be a dielectric film, and functionsto reduce the reflectance of an area that does not desire laseroscillation, so that the oscillation transverse mode can be controlled.

[0008] Japanese Laid-Open Patent Application Publication No. 9-326530proposes a VCSEL intended to reduce the jitter (fluctuation in theturn-on delay time) of the laser device. A diffusion reinforcementregion provided in the vicinity of the active layer is doped with anacceptor impurity of a high concentration, so that the number of holesinduced in the quantum well region is approximately one-digit largerthan the number of electrons therein. This increases the diffusion ratein the quantum well region.

[0009] However, the conventional selectively oxidization typesemiconductor lasers have the following problems to be solved. FIGS. 7Aand 7B respectively show a beam profile and an optical output profile ina far-field image obtained when the multimode surface emittingsemiconductor laser is used as an optical source. A symbol L denotes thedistance from the optical source to the far-field image, and θ denotesthe divergence angle of the diameter of the beam in the far-field imageviewed from the center of the optical source. Generally, this kind ofsurface emitting semiconductor laser has a profile like a doughnutshape. More particularly, the laser beam of the fundamental mode existswithin a spread or divergence angle θ₀ from the center of the opticalsource, and the beam profile is indicated by P0. When the divergenceangle is greater than θ₀, the resultant laser beam is of a high-ordermode in which the first-order mode or a higher-order mode in addition tothe first-order mode is combined with the fundamental mode. The beamprofile of the high-order-mode laser beam is indicated by P1. Theoptical output in the high-order mode indicated by P1 is grater thanthat of the fundamental mode. In other words, the outer portion P1 ofthe doughnut-like profile involved in the first-order or higher-mode isbrighter than the inner portion P0 involved in the fundamental mode. Thelaser emissions in the oscillation modes do not have the same profilesand do not occur concurrently. Thus, there is a difference in responsebetween the fundamental-mode light and the first-order or higher-orderlight. The difference in response causes the jitter in the opticalsignal when the multimode surface-emitting laser is used as the opticalsource.

[0010]FIG. 8A shows an eye pattern that serves as an index for checkingthe quality of the laser beam. The eye pattern is a pattern in which,when the laser beam is modulated (turned on and off), the modulatedlight signal is superimposed at random. The horizontal axis of the eyepattern denotes the time, and the vertical axis denotes the opticalpower. The eye pattern shown in FIG. 8A is an example observed when amultimode VCSEL capable of outputting light that has an optical power of3 mW and a wavelength of 850 nm is used as the optical source. It can beseen from the eye pattern of FIG. 8A that each emission in therespective oscillation mode does not occur concurrently, so that ajitter J takes place. The jitter J is the time difference in responsebetween the optical signals in the oscillation modes. The eye patternideally converges on a single line. If the optical output contains ajitter that exceeds a threshold time, the rate of incidence of errorcontained in the optical signal on the receive device side willconsiderably increase. This restrains the maximum bit rate achievable inthe data link.

[0011] The aforementioned transparent layer arranged in the vicinity ofthe beam emission aperture proposed in Japanese Laid-Open PatentApplication Publication No. 2001-156395 is not intended to reduce thejitter in the multimode laser beam but to control the oscillationtransverse mode. The VCSEL proposed in Japanese Laid-Open PatentApplication Publication No. 9-326530 does not have any mechanism forreducing the jitter.

SUMMARY OF THE INVENTION

[0012] The present invention has been made in view of the abovecircumstances and provides a surface emitting semiconductor laser.

[0013] According to an aspect of the present invention, there isprovided a surface emitting semiconductor laser comprising: a laminateof semiconductor layers emitting multimode laser light; and a blockmember blocking light of a specific mode among the multimode laser lightemitted from the laminate.

[0014] According to another aspect of the present invention, there isprovided a surface emitting semiconductor laser comprising: a surfaceemitting semiconductor laser capable of emitting multimode laser beam; apackage that houses the surface emitting semiconductor laser and has atransmission window via which the multimode laser light is emitted; anda block member that is provided in the transmission window and blockslight of a specific mode among the multimode laser light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Preferred embodiments of the present invention will be describedin detail based on the following figures, wherein:

[0016]FIG. 1A is a cross-sectional view of a surface emittingsemiconductor laser according to a first embodiment of the presentinvention taken along a line X1-X1 shown in FIG. 1B;

[0017]FIG. 1B is a plan view of the surface emitting semiconductor laseraccording to the first embodiment of the present invention;

[0018]FIG. 1C shows a relation between a block member and an emissionaperture in the first embodiment of the present invention;

[0019]FIG. 2A is a beam profile of the laser shown in FIGS. 1A through1C in a far-field image;

[0020]FIG. 2B is a relation between the divergence angle from an opticalsource and the optical output in the first embodiment of the presentinvention;

[0021]FIG. 3A is a plan view of a surface emitting semiconductor laserdevice according to a second embodiment of the present invention;

[0022]FIG. 3B is a cross-sectional view taken along a line X2-X2 shownin FIG. 3A;

[0023]FIGS. 4A, 4B and 4C show a part of a method of fabricating thesurface emitting semiconductor laser according to the first embodimentof the present invention;

[0024]FIGS. 5D, 5E and 5F show another part of the method of fabricatingthe surface emitting semiconductor laser according to the firstembodiment of the present invention;

[0025]FIGS. 6G, 6H and 6I show yet another part of the method offabricating the surface emitting semiconductor laser according to thefirst embodiment of the present invention;

[0026]FIG. 7A is a beam profile of a conventional surface emittingsemiconductor laser in a far-field image;

[0027]FIG. 7B is a relation between the divergence angle from an opticalsource and the optical output in the conventional surface emittingsemiconductor laser;

[0028]FIGS. 8A and 8B show eye patterns of laser beams; and

[0029]FIGS. 9A and 9B show optical communication systems according tothe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] A description will now be given of embodiments of the presentinvention with reference to the accompanying drawings.

[0031]FIG. 1A is a cross-sectional view of a VCSEL according to a firstembodiment of the present invention taken along a line X1-X1 shown inFIG. 1B that is a plan view of the VCSEL, and FIG. 1C shows a relationbetween the size of a block member and an emission aperture.

[0032] A multimode surface emitting semiconductor device 100 has ann-type GaAs substrate 1 on which laminated are an n-type lowermultilayer reflection film 2, an undoped active region 3, a currentconfinement layer 4, a p-type upper multilayer reflection film 5, ap-type contact layer 6, an interlayer insulation film 7, a p-type upperelectrode 8, and a block member 9 provided in the emission aperture 11.An n-type backside electrode 10 is provided on the backside of then-type GaAs substrate 1.

[0033] A cylindrical mesa or post 101 is formed on the substrate 1 sothat the post 101 ranges from the contact layer 6 to the active region3. The bottom of the post 101 is an exposed portion of the lowermultilayer reflection film 2. The interlayer insulation film 7 totallycovers the bottom and side of the post 101, and partially covers the topof the post 101. On the top of the post 101, the upper electrode 8 formaking an electrical contact with the contact layer 6 is disposed.Further, the upper electrode 8 defines the emission aperture 11, whichis a circularly-shaped aperture. The upper electrode 8 extends from thetop of the post 101 to the bottom thereof via the side, and is connectedto an electrode pad (not shown).

[0034] The circular block member 9 is placed on the emission aperture11. The block member 9 and the emission aperture 11 are concentricallyarranged, and the centers thereof coincide with the optical axis of thepost 101. The block member 9 should have an ability of blocking lightfrom the post 101, and may be made of a metal or another kind ofmaterial. Preferably, the block member 9 is made of the same material asthe upper electrode 8. In this case, the block member 9 and the upperelectrode 8 can be defined simultaneously. As will be described later,the block member 9 blocks light of a specific mode among the multimodelaser beams emitted from the post 101. The current confinement layer 4includes a conductive region formed by the AlAs layer and an oxideregion 4 a that surrounds the periphery of the conductive region. Theoxide region 4 a is formed by selectively oxidizing the AlAs layer fromthe side surface of the post 101. By controlling the oxidizationdistance from the side surface of the post 101, it is possible to obtaina desired aperture size suitable for the outer diameter of the post 101.In the present embodiment, in order to oscillate the multimode laserbeams, the current confinement layer 4 is designed so that theconductive region has a diameter D2 of 8 microns, and the emissionaperture 11 has a diameter D3 of 10 microns. The block member 9 isdesigned to have a diameter D1 of 3 microns. The diameter D1 of theblock member is selected so as to suitably block the laser beam of thefundamental mode among the multimode laser light, as will be describedlater. For example, the diameter (size) of the block member 9 is smallerthan that of the conductive region in the current confinement layer 4.

[0035] As has been described previously with reference to FIGS. 7A and7B, the selectively oxidization type multimode surface emittingsemiconductor laser generates the fundamental mode within the angle θ₀about the optical axis of the optical source, and generates ahigher-order mode at an angle larger than the angle θ₀. The opticalpower of the higher-order mode is greater than that of the fundamentalmode. The frequency response of the fundamental mode in the inner areaP0 of the doughnut-like profile is greater than that in the outer areaP1. In the present embodiment, since the block member 9 is provided inthe emission aperture 11, the light of the fundamental mode is blocked.The optical power of the fundamental mode is smaller than that of thelight of the higher-order mode. Thus, even if the fundamental mode iscutoff, the total optical power will not be greatly reduced. By shuttingout the light of the fundamental mode, the time difference in responsebetween the modes can be reduced, so that the jitter contained in thelaser beam can be reduced and the frequency response can be improved.

[0036]FIG. 2A shows a laser profile of the laser beam in a far-fieldimage with the block member 9, and FIG. 2B shows an output profilethereof. The light of the fundamental mode among the multimode laserbeams has an angle θ₀ in the far-field image at a distance L from theoptical source. The block member 9 is designed to have a diameter D1enough large to cover the angle θ₀, and may be 3 microns in the presentembodiment. Thus, the light of the fundamental mode among the beamsemitted from the post 101 is shut out by the block member 9, and theremaining beams are emitted via the emission aperture 11. The innerregion P0 of the doughnut-like profile is totally covered by the blockmember 9, while only light of the first-order or higher-order modelocated on the outer area P1 can be emitted.

[0037] The use of the block member 9 brings about an improved eyepattern as shown in FIG. 8B. As compared to the conventional laser inthe absence of any block member, according to the present embodiment,the eye pattern is converged and the jitter J1 is reduced.

[0038] A description will now be given of a surface emittingsemiconductor laser according to a second embodiment of the presentinvention with reference to FIGS. 3A and 3B. FIG. 3A is a plan view ofthe surface emitting semiconductor laser mounted on a metal stem (a caphas been removed from the stem), and FIG. 3B is a cross-sectional viewtaken along line X2-X2 shown in FIG. 3A.

[0039] A laser device 110 according to the second embodiment of thepresent invention does not employ the block member 9 in the emissionaperture 11 of a surface emitting semiconductor laser 110, but includesa block member in a transmission window of a casing body on which thesemiconductor laser is mounted.

[0040] The surface emitting semiconductor laser device 110 includes ametal stem 20, and a metal cap 30 attached to the metal stem 20. Themetal stem 20 is a thin plate of a circular shape, and has a ring-shapedflange 21 including a step on the circumferential periphery of the metalstep 20. The cap 30 has a single open side, and includes an insidehollow cylindrical space. The cap 30 has an end portion 31, which isformed on its circumferential periphery and horizontally protrudes fromthe side surface thereof. A circular transmission window (aperture) 32is formed in the center of the cap 30. A circular glass member 33 isattached to the back surface of the cap 30 so as to match the shape ofthe back surface. Preferably, the peripheral surface of the glass member33 is fused and fixed to the back surface of the cap 30. The glassmember 33 closes the transmission window 32 formed in the cap 30. Thetransmission window 32 serves as a path window via which the laser beamemitted from the post of the surface emitting semiconductor laser 120.The end portion 31 of the cap 30 is fixed to the flange 21 by welding orthe like. The cap 30 and the metal step 20 define an inside closedcavity, which is filled with nitrogen. A circular block member 34, whichis concentrically arranged with the transmission window 32, is fixed tothe glass member 33. The glass member 33 has a size enough large toblock the light of the fundamental mode among the multimode laser lightemitted from the semiconductor laser 120.

[0041] The metal stem 20 has through holes 22 and 23 to which a pair oflead pins 40 and 41 is attached. The pair of lead pins 40 and 41 isprovided with an insulating film 24 by which the lead pins 40 and 41 areelectrically isolated from the metal stem 20. Ends of the lead pins 40and 41 protrude from the surface of the metal stem 20. The protrudingportions of the lead pins 40 and 41 are connected to bonding pads 60 and61 by bonding wires 50 and 51.

[0042] A mounter 70 is attached to the center of the metal stem 20. Thesurface emitting semiconductor laser 120 is fixed to the mounter 70. Thesemiconductor laser 120 is the same as the semiconductor laser 100 shownin FIGS. 1A through 1C except that the laser 120 does not have the blockmember 9. The mounter 70 may be made of, for example, a semiconductormaterial such as GaAs or ceramics such as AlO (alumina) and AlN(aluminum nitride). The surface of the semiconductor or ceramic materialis plated with a metal such as Au. Alternatively, Au may be deposited onthe surface. The n-side electrode 10 of the semiconductor laser 120 isconnected to the metal surface of the mounter 70. Further, the electrodepad 61 is provided on the metal surface of the mounter 70. Furthermore,the semiconductor laser 120 includes the post 101 on the semiconductorsubstrate and includes the electrode pad 60 that is located close to thepost 101 and is electrically connected to the p-side electrode 8. Thelead pin 40 is electrically connected to the p-side electrode 8 via thebonding wire 50 and the electrode pad 60. The lead pin 41 iselectrically connected to the n-side electrode 10 via the bonding wire51 and the electrode pad 61.

[0043] The center of the block member 34 is located on the optical axisof the post 101. As is shown in FIGS. 2A and 2B, the diameter D1 of theblock member 34 can be defined as follows:

D1=L1×2 tan (θ₀/2)

[0044] where L1 is the distance from the post 101 serving as the opticalsource to the block member 34. In this case, the parameter θ₀ is thedivergence angle of the diameter of the beam of the fundamental modefrom the optical source (post) in the far-field image of the multimodelight emitted from the optical source. With this arrangement, the blockmember 34 blocks only the light of the fundamental mode among themultimode light emitted from the post 101.

[0045] The block member 34 may be made of a material capable of shuttingout light, such as a metal or insulator. Preferably, the surface of theblock member 34 is coated with an antireflection film in order toprevent light reflected by the block member 34 from traveling within thecap 30. An adhesive layer may be provided in order to facilitateadhesion of the block member 34 to the glass member 33. The block member34 may be an absorption member capable of light having a specificwavelength. In this case, the wavelength to be absorbed is thewavelength of emission by the semiconductor laser 120.

[0046] According to the second embodiment, after the surface emittingsemiconductor laser is mounted, the block member 34 is merely fixed tothe glass member 33 of the cap 30, so that the jitter-reduced multimodesurface emitting semiconductor laser having improved frequency responsecan be fabricated without any complex process. The combination of thesemiconductor laser thus obtained and the multimode optical fiber willachieve economical optical communication systems having a higher datatransmission rate.

[0047] A description will now be given, with reference to FIGS. 4A to4C, 5D to 5F and 6G to 6I, of a method of fabricating the surfaceemitting semiconductor laser according to the first embodiment of thepresent invention. Referring to FIG. 4A, the lower multilayer reflectionfilm 2, the active region 3, the p-type AlAs layer 4, the uppermultilayer reflection film 5, and the AlAs contact layer 6 are laminatedon the (100) surface of the semi-insulation GaAs substrate 1 in thatorder by MOCVD (Metal Organic Chemical Vapor Deposition). The lowermultilayer reflection film 2 includes a laminate of pairs of an undopedAl_(0.8)Ga_(0.2)As layer and an undoped Al_(0.1)Ga_(0.9)As layer. Theactive region 3 is a laminate of a spacer layer formed by an undopedAl_(0.4)Ga_(0.6)As, a barrier layer formed by an undopedAl_(0.2)Ga_(0.8)As layer, and a quantum well layer formed by an undopedGaAs layer. The upper multilayer reflection film 5 includes a laminateof pairs of p-type Al_(0.8)Ga_(0.2)As layer and a p-typeAl_(0.1)Ga_(0.9)As layer.

[0048] The lower multilayer reflection film 2 is composed of multiplepairs of an n-type Al_(0.9)Ga_(0.2)As layer and an n-typeAl_(0.15)Ga_(0.85)As layer. Each layer is λ/4n_(r) thick where λ is theoscillation wavelength and n_(r) is the refractive index of the medium.The paired layers having different composition ratios are alternatelylaminated to a thickness of 36.5 periods. The carrier concentrationafter the reflection film 2 doped with silicon as the n-type impurity is3×10¹⁸ cm⁻³.

[0049] The active region 3 includes a laminate of an 8 nm-thick quantumwell active layer formed by an undoped GaAs layer and a 5 nm-thickbarrier layer formed by an undoped Al_(0.2)Ga_(0.8)As layer, theselayers being alternately laminated. The outer layer of the laminate isthe barrier layer. The laminate is disposed in the middle of the spacerlayer formed by an undoped Al_(0.4)Ga_(0.6)As layer. The thickness ofthe spacer layer including the quantum well active layer and the barrierlayer is designed to be an integer multiple of λ/n_(r). The activeregion 3 thus formed emits 850 nm light.

[0050] The upper multilayer reflection film 5 is a laminate of pairs ofa p-type Al_(0.8)Ga_(0.2)As layer and a p-type Al_(0.1)Ga_(0.9)As layer.Each layer is λ/4n_(r) thick. The paired layers having differentcomposition ratios are alternately laminated to a thickness of 22periods, which include the underlying AlAs layer 4 and the uppermostGaAs layer 6. The AlAs layer 4 is not required to be AlAs throughout thethickness λ/4n_(r) but may include anther semiconductor layer. If theAlAs layer is too thick, optical scattering loss may increase. In thepresent invention, the AlAs layer 4 is 30 nm thick, and the rest isformed of Al_(0.9)Ga_(0.1)As. The carrier concentration after the layer5 is doped with carbon as the p-type impurity is 4×10¹⁸ cm⁻³.

[0051] In order to reduce the series resistance of the device, anintermediate layer may be placed between the Al_(0.8)Ga_(0.2)As layerand the Al_(0.1)Ga_(0.9)As layer of the upper multilayer reflection film5, wherein the intermediate layer has an Al composition ratio betweenthese layers.

[0052] The uppermost layer of the upper multilayer reflection film 5 isthe p-type GaAs layer that is 20 nm thick in order to improve thecontact with the p-side electrode 8. The upper multilayer reflectionfilm 5 has a carrier concentration of 1×10¹⁹ cm⁻³ after it is doped withzinc as the p-type impurity.

[0053] Next, as is shown in FIG. 4B, the substrate (wafer) is removedfrom the growth chamber, and a mask pattern 13 of SiO₂ isphotolithographically formed on the substrate. With the SiO₂ patternbeing used as a mask, the substrate is etched so that the cylindricalmesa or post 101 is formed thereon, as shown in FIG. 4C. Then, the uppermultilayer reflection film 5, the AlAs layer 4 and the active region 3are anisotropically etched by reactive ion etching (RIE).

[0054] After the post 101 is formed, the side surface of the AlAs layer4 is exposed. The substrate (wafer) is exposed to a vapor atmosphere at350° C. with nitrogen being used as carrier gas (flow rate: 2 liters perminute) for 30 minutes. The AlAs layer 4, which forms a part of theupper multilayer reflection film 5, has an oxidization rate that is veryhigher than that of the Al_(0.8)Ga_(0.2)As layer or Al_(0.1)Ga_(0.9)Aslayer which layers form a part of the AlAs layer 4. As shown in FIG. 5D,oxidization starts from the side surface of the AlAs layer 4 just abovethe active region 3 included in the post 101. This oxidizationeventually makes the oxide region 4 a that reflects the shape of thepost 101. The oxide region 4 a has a reduced conductivity and serves asa current confining portion. Further, the oxide region 4 a has anoptical reflectance (˜1.6) that is approximately half the reflectance ofthe layers, and thus serves as a light confining region. The rest of theAlAs layer 4 that has not been oxidized serves as a conductive regionand functions as a current injection part. The aperture of theconductive region has a diameter as large as 10 microns for emission ofmultimode light.

[0055] Thereafter, as shown in FIG. 5E, the mask 13 is removed and theexposed side surface of the post 101 is coated with the insulation film7 that also covers the substrate. Then, as shown in FIG. 5F, a contacthole 7 a is formed in the top of the post, so that the interlayerinsulation film 7 can be defined.

[0056] Then, as shown in FIG. 6G, an electrode layer is provided on theentire substrate (wafer) including the post 101, and is then patterned,as shown in FIG. 6H. The p-side electrode 8 defines the emissionaperture 11 on the top of the post 101, and simultaneously, the blockmember 9 is formed. After that, as shown in FIG. 6I, the n-sideelectrode 10 is formed on the backside of the substrate 1. In theabove-mentioned manner, the surface emitting semiconductor laser shownin FIGS. 1A through 1C can be produced. The surface emittingsemiconductor laser 110 according to the second embodiment of thepresent invention, the p-side electrode is patterned so that the blockmember 9 does not remain.

[0057]FIGS. 9A and 9B show optical communication systems equipped withthe surface emitting semiconductor laser according to the presentinvention. More particularly, FIG. 9A shows an optical communicationsystem in which a surface emitting semiconductor laser 91 of theinvention is directly coupled to an optical fiber 92. FIG. 9B showsanother optical communication system in which the surface emittingsemiconductor laser 91 is coupled with the optical fiber 92 via a lens.

[0058] The present invention is not limited to the specificallydisclosed embodiments, and variations and modifications may be madewithout departing from the scope of the present invention.

[0059] In the first and second embodiments of the invention, the lightof the fundamental mode is blocked. Alternatively, light of anotherspecific mode may be blocked to reduce the jitter contained in the lightsignal. Preferably, modes that have a relatively low frequency responseare selected. The shape of the block member may be selected inaccordance with the mode to be cutoff. The material of the block membermay be metal or insulator. It is essential to select a material that canblock light of the selected mode. In this regard, the block member maybe made of a material that can absorb light of the selected wavelength.

[0060] According to the present invention, light of the specific mode isblocked or cutoff by means of a member provided in the emission apertureof the surface emitting semiconductor laser capable of emittingmultimode laser light. It is therefore possible to reduce jittercontained in the light signal and improve the frequency response and totransmit data at a higher bit rate.

What is claimed is:
 1. A surface emitting semiconductor lasercomprising: a laminate of semiconductor layers emitting multimode laserlight; and a block member blocking light of a specific mode among themultimode laser light emitted from the laminate.
 2. The surface emittingsemiconductor laser as claimed in claim 1, wherein: the laminatecomprises a substrate, a lower reflection mirror on the substrate, anupper reflection mirror, an active region, and a current confinementlayer, the active region and the current confinement layer beinginterposed between the upper and lower reflection mirrors; and the blockmember is provided in an emission aperture provided above the upperreflection mirror.
 3. The surface emitting semiconductor laser asclaimed in claim 1, wherein: a top of the laminate is partially coveredwith an electrode so that an emission aperture can be defined; and theblock member is provided on the top of the laminate and is located inthe center of the emission aperture.
 4. The surface emittingsemiconductor laser as claimed in claim 1, wherein: the laminate has amesa; an emission aperture is formed on the mesa; and the emissionaperture and the block member have shapes related to an outer shape ofthe mesa.
 5. The surface emitting semiconductor laser as claimed inclaim 1, wherein: the laminate comprises a current confinement layerhaving a conductive region; and the block member has a size smaller thana size of the conductive region.
 6. The surface emitting semiconductorlaser as claimed in claim 1, wherein the block member and the electrodeare simultaneously formed.
 7. The surface emitting semiconductor laseras claimed in claim 1, wherein the block member blocks light of afundamental mode among the multimode laser light.
 8. A surface emittingsemiconductor laser comprising: a surface emitting semiconductor lasercapable of emitting multimode laser beam; a package that houses thesurface emitting semiconductor laser and has a transmission window viawhich the multimode laser light is emitted; and a block member that isprovided in the transmission window and blocks light of a specific modeamong the multimode laser light.
 9. The surface emitting semiconductorlaser as claimed in claim 8, wherein the block member has a diameter D1defined as follows: D1=L1×2 tan (θn/2) where L1 is a distance from thesurface emitting semiconductor laser device to the block member, θn is adivergence angle of emitted light of the specific mode from an opticalsource of the surface emitting semiconductor laser device in a far-fieldimage of the multimode laser light.
 10. The surface emittingsemiconductor laser as claimed in claim 8, wherein the block member isequipped with an antireflection film that prevents the multimode laserlight from being reflected by the block member.
 11. The surface emittingsemiconductor laser as claimed in claim 8, wherein the block memberincludes an absorption member that absorbs the light of the specificmode.
 12. The surface emitting semiconductor laser as claimed in claim8, wherein: the surface emitting semiconductor laser device has a mesathat emits the multiple laser light; and the block member has a circularshape.
 13. The surface emitting semiconductor device as claimed in claim9, wherein: the block member blocks of light of a fundamental mode amongthe multimode laser light; and the divergence angle En is a divergenceangle of a diameter of the light of the fundamental mode from an opticalsource of the surface emitting semiconductor laser device.
 14. Anoptical communication system comprising: a surface emittingsemiconductor laser; and an optical fiber optically connected to thesurface emitting semiconductor laser, the surface emitting semiconductorlaser comprising: a laminate of semiconductor layers emitting multimodelaser light; and a block member blocking light of a specific mode amongthe multimode laser light emitted from the laminate.
 15. An opticalcommunication system comprising: a surface emitting semiconductor laseremitting multimode laser light; and an optical fiber optically connectedto the surface emitting semiconductor laser, the surface emittingsemiconductor laser comprising: a package that houses the surfaceemitting semiconductor laser and has a transmission window via which themultimode laser light is emitted; and a block member that is provided inthe transmission window and blocks light of a specific mode among themultimode laser light.